The Lac St-Jean anorthosite massif underlies an area of over 20,000 km 2 and has been emplaced into migmatitic gneisses of the central granulite terrain of the Grenville Province of the Canadian shield.
Field data and petrography in an area straddling the anorthosite-gneiss contact, close to Chicoutimi (Quebec) permits an outline of its tecto-magmatic evolution. Depositional magmatic textures in the massif reveals that it crystallized from a magma
in a relatively calm tectonic environment. The absence of fusion in pelitic gneisses at the contact proves that the crystallization did not take place at the level presently exposed. The parallelism of subvertical foliation in the enveloping gneisses
and the anorthosite indicates that both were deformed together. It is suggested that the deformation results from a diapiric ascent of the anorthosite massif after its consolidation at depth. The depth of consolidation of the anorthosite is estimated
at ~25 30 km from subsolidus reaction between plagioclase and olivine. The diapiric ascent is further substantiated by the fact that three sets of mafic dykes of different ages, intrusive into the anorthosite, have a mineralogy which indicates
successively decreasing P, T conditions of emplacement from granulite facies to amphibolite facies. An evolution of the basement gneisses and the anorthosite is proposed as a working hypothesis; it relies on the fact that metabasite dyke swarms in
the basement gneisses represent a period of major crustalextension and could be used as a stratigraphic subdivision of the Grenville Province.